There is known a locking system (“smart entry system”) configured to effect communication between a portable or mobile device carried by a user and a main apparatus mounted on a vehicle to verify the mobile device and control automatic locking/unlocking of a door. U.S. No. 2007/0216175A1, in its detailed disclosure (Patent Document 1), describes a technique relating to a door handle of a vehicle for executing locking/unlocking of a door based on detection of a user's operation on the door handle, after verification of a mobile device executed between a vehicle mounted apparatus and the mobile device. The door handle incorporates therein e.g. a capacitance detection apparatus for detecting approaching or touching of a user's hand to/with the door handle (Patent Document 1: see e.g. paragraphs [0025]-[0030]).
The capacitance detection apparatus is comprised of a switched capacitor technique which is described in e.g. the detailed disclosure of U.S. Pat. No. 7,015,705 B2 (Patent Document 2). According to this technique, a period for keeping a switch closed between a capacitance to be measured (“measured capacitance”) and a reference capacitance is set to a certain multiple (several-fold) of a time constant which is represented by a product of an on-resistance of the switch and the capacitance to be measured. This arrangement serves to reduce deterioration in the sensitivity in the case of presence of a leak resistance in the measured capacitance, which occurs under a high-humidity environment, inadvertent adherence of water drops or the like to the surface of the capacitance detection apparatus, etc. (Patent Document 2: see e.g. column 5, L16 to column 7, L17, etc.).
The capacitance detection apparatus of Patent Document 2 is a distinguished apparatus capable of alleviating the effect of sensitivity deterioration in the presence of leak resistance in the measured capacitance. However, at the very moment of water adherence to the apparatus, the apparatus may be unable to effectively discriminate this adherence of water from a touching by a human. For instance, with the capacitance detection apparatus mounted to a door handle of a locking system disclosed in Patent Document 1, in the event of splashing of water onto the apparatus due to raindrops or car washing operation, locking/unlocking of the door may take place inadvertently, irrespective of the user's locking/unlocking intension.